One of the more troublesome factors affecting the reliability and life of welding power supplies is foreign matter pulled into the system by the cooling air. To circumvent this condition, power supplies can be designed for convection cooling, e.g. without forced air; or they can be sealed and cooled by various methods. Usually, either approach increases the size, or the weight, or the cost, etc.
For example, welding power supplies have been in the past immersed in a liquid (such as transformer oil) and sealed. In such case, heat from the components is transferred through the oil to the case then to the atmosphere. In order to increase the effective thermal exchange area, the container may be finned.
Some of the cooling methods used in the past are:
1. Sealed power supply with derated power.
2. Sealed power supply with internal air circulation to transfer heat from the components to the case; also with additional air flow over the external surface of the case for increased heat dissipation.
3. Recirculation of the air through a water to air heat exchanger with removal of the heat by the cooling water. The water system may be part of or external to, a large central supply or it may be recirculated in a closed loop system. Occasionally, two heat exchangers have been employed: one inside the sealed power supply taking heat from the components; one outside the power supply removing heat from the circulated liquid (or gas).
4. Direct cooling of the components by liquid in a sealed case, the cooling liquid being in and out, or recirculated.
5. Sealed components cooled by a refrigeration system with evaporator inside the sealed welder and the condenser and compressor external to it.
All of these methods would protect the power supply from foreign matter, moisture, chemical atmospheres, etc. However, all are costly and in addition they forfeit the advantage of portability. Some compromise has been possible to provide longer life at a reasonable increase of the cost. For example, if the sealed power supply approach is chosen, a 500 amp machine has to be derated by a factor of approximately 4. The user installs equipment rated for 500 amps, which can only be operated at much less over an extended time period.
On the other hand, if the user settles for less reliability, the equipment with shorter life will cost more in the long term, all costs being considered. More costs are also involved to install, trouble shoot, and repair with the inevitable loss of production.
Hostile environment is a major problem with arc welding apparatus by reason of the conditions of work by welding. A study of field failures primarily in coastal atmospheres, or for chemical or petro chemical plants and, paper mills, or on construction sites, has shown that power transformers dipped and baked with insulating varnish hold up well in the hostile environments. So does a metal sheet that has received a coat of varnish during the transformer insulating process, whereas metal sheets with only standard finish would rust badly. Terminal boards when clogged with foreign matter and covered by moisture often "arc over". Destruction of the board and damage to the main transformer and other control components ensue. Exposed copper to copper, or copper to aluminum electrical connections would corrode severely. Relays, contactors, switches exposed to dirt and moisture often fall electrically and mechanically. Rough handling is also a common cause of damage to welder cases.